Field of the Invention
The present invention relates to a self-tapping screw for soft metals that is used for soft metals, such as aluminum alloys and magnesium alloys.
Description of the Invention
A self-tapping screw is a screw that can be driven into a pilot hole formed in a mating member, so that it can be tightened while creating a female screw on the inner surface of a pilot hole. The self-tapping screw has been widely used because of its feature of requiring no tapping process in advance in the pilot hole.
The self-tapping screw has been required to be capable of being driven by minimum possible initial driving torque. In this context, one commercially available self-tapping screw has a shaft with a polygonal cross section achieving a smaller contact area with the pilot hole as described in Patent Literature 1.
Unfortunately, there has been a problem in that such a self-tapping screw having a non-circular cross section needs to be manufactured with an intricate and expensive mold, and thus requires a higher manufacturing cost than standard screws. In addition, the shaft of the self-tapping screw having a non-circular cross section has a male screw that does not mate with the female screw as the mating member evenly over the entire circumference even in the fully tightened state. More specifically, the male screw tightly mates with the female screw at the apexes of the polygon, and not at a portion between the apexes. All things considered, there has been a problem in that axial force in the tightened state is more likely to be reduced due to heat or the like, compared with standard screws having circular cross sections.
Conventional self-tapping screws are supposed to be mated with a mating member made of steel, and thus have sharp cutting edges as described in Patent Literature 2. Thus, when such a screw is driven into soft metals, such as aluminum alloys and magnesium alloys, a large amount of chip powder is produced due to excessive tapping. In particular, when the mating member is made of a magnesium alloy, chip powder produced might catch fire in a certain environment.